Milk fat of humans and other species contains significant quantities of triglycerides with medium chain fatty acids, is readily digestible, and provides the neonate with a major energy source at a time when its lipid-digestive apparatus has not fully matured. The ingestion of milk fat by the neonate induces a mild ketosis and these ketones are apparently important for growth and development of the brain. Our program is directed toward elucidating the mechanisms which regulate the production of this important nutrient by the mammary gland. We have shown, with rats, that the biosynthesis of medium chain fatty acids requires two enzymes, fatty acid synthetase and thioesterase II. These enzymes appear asynchronously in developing mammary gland. The level of thioesterase II rises to a maximum by late pregnancy and correlates closely with the potential of the gland for medium chain fatty acid synthesis; the level of fatty acid synthetase rises only after parturition and correlates closely with the overall lipogenic capacity. Since thioesterase II rises at a time when epithelial cell proliferation occurs, it is not clear whether the enzyme is constitutive to the epithelial cells or whether it is the product of cellular differentiation. We propose to isolate epithelial cells from rats at different stages of development to determine whether the level of thioesterase II changes on a "per cell" basis. Hormonally manipulated animals will be used to determine which hormones are responsible for the increased level of thioesterase II observed during pregnancy. Assay systems will be developed to detect the enzyme in human mammary epithelial cells obtained from milk, with the aim of evaluating factors which may influence the level of this enzyme and consequently the content of medium chain fatty acids in human milk.